1. Field of the Invention
The invention relates to a display device, and a method for repairing a defective pixel, in particular, a method capable of repairing a defective pixel easily even if the pixel is in a constantly non-lit state.
2. Description of the Related Art
In recent years, organic EL display devices have been developed wherein pixels are made of organic EL elements (organic electroluminescence elements). Organic EL elements have characteristics that the elements emit light, the elements may be made very thin, the elements are light weight, and the elements have a wide viewing angle and a high-speed responsibility. Thus, the organic EL display devices are expected as the next-generation thin display devices. However, as the performance of liquid crystal display devices has become higher and the cost thereof has become lower, the performances and the price required for organic EL display devices have been becoming severer. In particular, from the viewpoint of costs, organic EL display devices are more expensive than liquid crystal display devices, the mass-producing technique of which has been established. Thus, it is urgently necessary to reduce the costs thereof.
One of factors of an increase in costs of organic EL display devices is that the yield thereof is low. Factors of lowering the yield of organic EL display devices are various. In particular, the yield is lowered by defects of pixels in many cases. The pixel defects are caused by the generation of a short circuit based on dust or dirt, the contact of a mask for dividing pixels into different colors with the pixels, a failure in light exposure based on dust or dirt. Owing to such pixel defects, pixels in a screen partially come not to emit light, or partially are in a constantly lit state which is unable to be controlled.
A device is deemed defective even when only a a minority of pixels in a screen are defective as described above. For this reason, methods of preventing the contact of pixels with dust or dirt, or a mask, thereby decreasing defects are known. However, it is difficult to remove defective pixels completely, and a great improvement in yield is not easily attained. Thus, the following are suggested as a method for repairing defective pixels: a method of applying a high-voltage pulse to a short-circuited moiety to make the moiety electrically nonconductive, thereby restoring the pixels therein to normal pixels (see Japanese Patent Application Laid-Open (JP-A-) No. 11-162637); and a method of radiating a laser ray to defective pixels to repair the pixels (see, for example, JP-A Nos. 2007-42498 and 2006-323032). When an electric current is constantly supplied to an EL element due to, for example, a short circuit of a driving transistor so that the EL element is in a constantly lit state, only a current-supplying line for the pixel made of this EL element is cut by a laser, thereby changing the constantly lit pixel to a constantly non-lit pixel and making the defect inconspicuous. However, if an image appears on the screen in which all pixels adjacent thereto are luminous and with a high brightness, the constantly non-lit pixel conversely becomes a conspicuous defect.
Suggested are also a method of setting up, for an expected defect, a preliminary storage capacitance or switching element, and a method of constructing pixels prepared for a case where some of the pixels become defect pixels (see, for example, JP-A Nos. 2005-92154 and 2003-15549). However, according to such a method, it becomes necessary to prepare extra defect-overcoming measures also for normal pixels, which are most of the entire pixels. As a result, the number of the producing steps increases to raise costs, the definition of the pixels deteriorates, the pixel numerical aperture lowers, and other inconveniences are caused. Thus, the method is not an easy method. Additionally, no countermeasures are taken for pixel defects based on an unexpected inferiority; thus, the defects may not be repaired.